High resolution pattern-transfer apparatus are required for the production of small-geometry integrated circuits. While pattern-transfer apparatus using optical radiation are widely used, charged-particle-beam ("CPB") apparatus using electron beams have also been used. CPB apparatus generally provide a higher resolution than that available with apparatus using optical radiation.
Integrated circuits are generally formed on a semiconductor wafer by forming a series of circuit patterns on a surface of the wafer. The circuit patterns are formed by transferring a sequence of circuit patterns sequentially onto the wafer surface. The resulting integrated circuit is a multi-layered structure in which planar circuits are layered parallel to the surface of the wafer.
In order to produce small-geometry integrated circuits, circuit patterns must be transferred with a high resolution and the various circuit patterns must precisely align on the wafer. Therefore, the individual circuit patterns must be correctly focused on the surface of the wafer and must be precisely registered with other circuit patterns. Registration errors must be kept within an acceptable tolerance.
In CPB pattern transfer, the mask defining the circuit pattern to be transferred to the wafer is usually divided into a plurality of smaller regions referred to as subfields; the pattern portion in a subfield is transferred to the wafer in a single exposure. The subfields are transferred so that they are accurately joined or "stitched" together on the wafer. One CPB system that stitches together subfield patterns is described in Japanese Kokai patent document No. HEI 8-64522.
In CPB pattern-transfer apparatus, aberration correction systems such as deflectors or dynamic compensators are provided to reduce image blurring and distortion in the subfield images caused by aberrations in the CPB optical system. The CPB optical systems use either electromagnetic or electrostatic lenses and high resolution is obtained by designing the deflectors and dynamic compensators to compensate the aberrations of the lenses.
Some pattern-transfer errors arise from variations in wafer thickness or rotational errors of the wafer with respect to the mask during exposure. These errors show up as rotational misalignments of a subfield image, magnification errors of the subfield image, and focus errors. With reference to FIG. 19A, illustrative subfield images a1, a2 are shown with respect to ideal subfield images b1, b2, respectively. The subfield images a1, a2 are tilted by an angle .theta. with respect to the ideal subfield images b1, b2 and are thus rotationally misaligned with respect to the ideal subfield images b1, b2. With reference to FIG. 19B, illustrative subfield images a1, a2 are shown with respect to ideal images b1, b2. The subfield images a1, a2 exhibit a magnification error, i.e., the subfield images a1, a2 are too large. In addition, the subfield images a1, a2 overlap along a seam c. With reference to FIG. 19C, subfield images are focused at a focal plane f that is a distance .DELTA.z above a wafer surface W. Thus, FIG. 19C illustrates a focus error. To correct the types of errors shown in FIGS. 19A-19C, the lens currents (or voltages if electrostatic lenses are used) are adjusted. Alternatively, one or more correcting lenses can be provided.
With reference to FIG. 20, a conventional electron-beam pattern-transfer apparatus includes an electron gun 901 that produces an electron beam EB. The electron beam EB propagates along an axis AX and is shaped into a desired transverse profile (e.g., a square) by an aperture 903. A condenser lens 902 then directs the electron beam EB to a selected subfield 951 of a reticle (mask) 905 with a subfield-selection deflector 904. The electron beam EB transmitted by the reticle 905 is then deflected by a deflector 908 and imaged with a predetermined demagnification onto the wafer 911 with projection lenses 909, 910. Deflector controllers 917, 918 control the magnitude and direction of the deflection produced by the deflectors 904 and 908, respectively.
The reticle 905 and wafer 911 are mounted on a reticle stage 906 and a wafer stage 912, respectively, that provide translations in an xy-plane as directed by respective stage controllers 907, 913. The locations of the stages 906, 912 are detected with corresponding position detectors 914, 915, typically, laser interferometers. A main controller 916 controls positioning so that the deflectors 904, 908 and stages 906, 912 are controlled based on the positions measured by the position detectors 914, 915.
With reference to FIG. 22, the mask 905 is divided into a plurality of subfields 951.sub.1 -951.sub.n, separated from each other by boundary regions 952 that either block or scatter the electron beam EB. The electron beam EB transmitted by a subfield such as the exemplary subfield 951.sub.1, that is displaced from the axis AX is imaged onto the wafer 911 at a corresponding transfer subfield 9111.sub.1, that is also displaced from the axis AX. The remaining subfields 951.sub.2 -951.sub.n are similarly projected onto corresponding transfer subfields 9111.sub.2 -9111.sub.n so that the circuit pattern is defined by the mask 905 transferred to a wafer field 9110.
The mask subfields 951 are separated by boundary regions 952 that are not transferred to the wafer 911. To prevent such transfer, appropriate deflection of the electron beam EB is controlled by the deflector 908. The mask subfields 951 are projected onto the wafer 911 such that the corresponding transfer subfields 9111 contact each other along their respective edges.
Unfortunately, aberration-correcting deflectors and dynamic compensators that correct CPB optical-system aberrations introduce additional aberrations. With reference to FIG. 21A, a portion of a circuit pattern that extends across transfer subfields 9111a and 9111b ideally joins accurately along a seam 9111c located between adjacent transfer subfields. Thus, conductors P extending from the subfield 9111a to the subfield 9111b extend cleanly and contiguously across the seam 9111c. Referring to FIG. 21B, if there is distortion in the CPB optical system, then subfield images Q1, Q2 are distorted, creating a gap Q3 between the subfield images Q1, Q2 and a corresponding break in the conductors P. With reference to FIG. 21C, distortion can also cause subfield images Q1, Q2 to overlap each other.
A CPB optical system comprises a series of electromagnetic or electrostatic lenses, each of which can exhibit manufacturing errors. In addition, these lenses also exhibit mounting errors so that the electromagnetic fields that focus and deflect the electron beam deviate from design values. The aberrations in CPB images are a function of the CPB path and the electromagnetic fields along the path. See, e.g., Chu and Munro, Optik 61:121-145 (1982). If the CPB optical system exhibits such manufacturing errors, aberrations such as defocus and distortion are introduced.
Dynamic correction of deflection aberrations has been achieved using astigmatism compensators comprising focus-correction coils or octopoles to reduce deflection image-plane distortions and deflection astigmatism. For example, X. Zhu et al., SPIE Proceedings, 2522:66-77 (1995) proposed using focus-correction coils and astigmatism compensators as an astigmatism corrector for 3rd-order deflection distortion and hybrid distortion aberrations. The apparatus of X. Zhu et al. uses two focus-correction coils and two astigmatism compensators and requires precise positioning. In some cases, to correct manufacturing errors, the positions of focus correctors and astigmatism compensators are recalculated and adjusted to reduce the aberrations. However, such readjustment is difficult and impractical.
Therefore, aberration correction methods and CPB apparatus are needed in which aberrations are reduced without a need for mechanical adjustment.